A new study has used skull anatomy to show that the evolution of birds from dinosaurs may have resulted from a drastic change in dinosaur development.

In a study published this week in the journal Nature, researchers from Harvard University, the Museum, the New York Institute of Technology, The University of Texas at Austin, and the Autonomous University of Madrid report evidence that while many dinosaurs took years to reach sexual maturity, birds sped up the developmental clock, which led them to retain the physical characteristics of baby dinosaurs.

The paper adds another dimension to a long history of research on the theropod origin of birds. “By analyzing fossil evidence from skeletons, eggs, and soft tissue of bird-like dinosaurs and primitive birds, we’ve learned that birds are living theropod dinosaurs, a group of carnivorous animals that include Velociraptor,” says Mark Norell, chair of the Museum’s Division of Paleontology and one of the paper’s co-authors. “This new work advances our knowledge by providing a powerful example of how developmental changes played a major role in the origin and evolution of birds.”

Although dinosaurs had mouths bristling with teeth and birds have proportionally larger eyes and brains, the skulls of modern birds and juvenile dinosaurs demonstrate a surprising degree of similarity, a realization that sparked this study.

Researchers used CT scanners to image dozens of skulls of species ranging from modern birds to theropods to early dinosaus. By noting various “landmarks”—such as the orbits, cranial cavity, and other bones in the skull—on each scan, scientists were able to track the changes in skull shape over millions of years.

The study revealed that while non-avian dinosaurs (even those closely related to modern birds) undergo vast morphological changes as they mature, the skulls of juvenile and adult birds remain remarkably similar. This “paedomorphosis,” in which the adults of a species look like the juveniles of their ancestors, was an important factor in the evolution of birds from dinosaurs.

Modern bird skulls resemble those of young dinosaurs due to a process known as progenesis, which causes an animal to reach sexual maturity earlier. Unlike their dinosaurian ancestors, modern birds take dramatically less time—just 12 weeks in some species—to develop to maturity, leading birds to retain the characteristics of their juvenile ancestors into adulthood.